An optical add drop multiplexer (OADM) is a device used to extract a set of optical signals (also called “wavelengths” or “channels” herein) from a wavelength division multiplexed (WDM) signal input to the OADM, and to subsequently reinsert the extracted set of wavelengths output by the OADM. FIG. 1 shows a block diagram of an OADM 100. A WDM optical signal comprises a plurality of wavelengths or channels. One of the wavelengths λ2, is extracted (also called “dropped” herein) from an input line side 102 via a drop port 104 and then subsequently reinserted (also called “added” herein) λ2 onto an output line side 108 via an add port 106. The purpose of adding and dropping wavelength(s) in this manner is to obtain information encoded on the dropped wavelength. New information may also be transmitted the added wavelengths. In most instances the carrier wavelength of the dropped wavelength is the same as carrier wavelength of the added wavelength.
OADMs may be implemented in a wide variety of different architectures and technologies. For example, one architecture involves arrayed waveguide grating routers and 2×2 optical switches. Another architecture involves a pair of interference filters that serve as multiplexers and demultiplexers. Depending on the architecture and the technology that is employed, an OADM may or may not be configurable, i.e., the determination of which wavelengths are dropped and added may or may not be fixed at the time of manufacture.
OADMs are employed in the network nodes of WDM transmission systems such as ring networks so that incoming data may be either passed through the node or dropped to a local receiver. If data from a particular wavelength is dropped, this wavelength is now available on the outbound direction, and hence new data can be added from a local transmitter. In a WDM system, when the optical signals are transmitted over long distances, periodic amplification of the optical signals is necessary to overcome fiber loss in the transmission path. Currently, amplification is accomplished by using optical amplifiers, e.g. Erbium Doped Fiber Amplifiers (EDFAs) or Raman amplifiers.
In general each of the wavelengths in a WDM transmission system employing optical amplifiers should have the same power. If the power levels of the wavelengths are not the same, those wavelengths having more power tend to be amplified more than other channels and take away gain that would otherwise be available for adjacent wavelengths. When such unbalanced wavelengths propagate through a series of optical amplifiers, deleterious effects may arise such as a high level of cross talk between adjacent wavelengths and nonlinearities in the fiber.
Accordingly, it would be desirable to provide an automatic power balancing arrangement for an OADM in which wavelengths or channels being added have the same power as the remaining wavelengths or channels in the WDM optical signal.